Cells may include one or more sectors. A cell without multiple sectors is a single sector cell, i.e., it includes a single sector. Signals are normally transmitted by a sector transmitter using a carrier frequency and the corresponding bandwidth, e.g., one or more tones surrounding the carrier frequency. Different cells and/or sectors of a cell often use different frequency bands centered around a carrier frequency used by the sector or cell. The carrier frequency of adjacent cells and/or sectors are often different. To receive signals corresponding to a carrier frequency, a wireless terminal normally has to adjust its receiver, e.g., receiver filters, to correspond to the frequency band associated with the carrier frequency to be used. Switching a receiver between carrier frequencies may take time. Thus, in receivers with a single filter chain, transitioning between different carriers may cause the receiver to encounter intervals during which information can not be received due to the switching process.
Wireless terminals, e.g., mobile nodes, communicating with a base station on a given carrier frequency and moving through a multi-carrier system need to decide when to make a handoff and transition to a new carrier frequency e.g., corresponding to a new cell and/or sector. As discussed above, an adjacent sector and/or cell may use a different carrier frequency, and as a sector or cell boundary is crossed, a wireless terminal will normally have to identify and switch to the new carrier frequency.
Typically a mobile node includes a single receiver chain and listens to one carrier frequency band at a given time due to constraints in the hardware and cost associated with the receiver. This is because, for cost reasons, multiple parallel receiver filter chains are often too expensive to be practical. In some known systems a mobile node waits until communications are lost or significantly degraded on the operating carrier band being used before switching to another carrier. In some systems, a wireless terminal periodically switches its receiver to a different carrier band to check for signal presence and/or strength. Unfortunately, while switched to search for another carrier, the receiver can not receive signals from the carrier that is currently in use. The known methods of determining what carriers are available to switch to and when to switch to a new carrier may result in interrupted communications, gaps during the hand-off process, and/or wasted resources in monitoring and determining the appropriate carrier frequency band.
In addition to the problem of determining which carriers/frequency bands are available and should be used at any given time, handoffs between sectors and/or cells using different carriers present problems associated with adjusting receiver and/or transmitter circuitry to switch between carrier frequencies. Problems associated with switching between carrier frequencies occur when a switch between carriers occurs whether or not a change in location occurs and are generally encountered when handoffs occur between carrier frequencies. For cost reasons, it is often desirable to implement a communications device with a single receiver and transmitter.
When switching between carrier frequencies, an analog filter used by the receiver and an analog filter used by the transmitter normally has to be changed to match the new frequency band. This normally involves adjusting the filter as a function of the carrier frequency of the new sector or cell. The transitional period required to implement this filter change, in the case of a device with a single receiver/transmitter results in an interval during which the communications device is unable to receive and/or transmit information to a base station.
In systems where each cell/sector uses the same frequency, e.g., in systems with a frequency reuse rate of 1, handoffs between sectors and/or cells do not require such filter switching operations since the frequency band used in each of the sectors/cells is the same. In such systems “make before break” handoffs are relatively easy to implement. In a make before break handoff the communications device directly communicates with the new sector and/or cell before breaking, e.g., terminating, the connection with the old base station. Given that the carrier frequencies are the same before and after handoff in such systems, there is normally no need to alter the filters in the receiver and/or transmitter circuitry making the time required to switch between the two sectors and/or cells relatively minor.
Regardless of whether a handoff operation involves a change in carrier frequency or not, in many systems when handing off from one base station or sector to another before a mobile device is permitted to transmit user data, e.g., application layer data such as voice, text, etc., the mobile node performs timing and/or power control synchronization operations. Registration in the sector or cell being entered is normally also required before transmission of user data to the new base station or sector is permitted. Such signal level synchronization operations can be important to prevent transmission by the mobile device entering the cell and/or sector interfering with the transmissions from other mobile devices already in the cell/sector being entered. In some systems, a particular period of time is set aside on a periodic basis for use by mobile devices entering the system to transmit signals used to register and/or perform initial timing and/or power control synchronization operations. During such periods of time, devices entering the cell/sector can contact the base station to perform timing and/or power control synchronization operations without interfering with devices already in the system, e.g., because registered devices know not to transmit signals during this particular period of time. Signaling during this dedicated period of time is often contention based, e.g., one or more new devices may attempt to register using the same communications resource, e.g., set of tones. In such cases, signals may collide and the registration by the devices attempting to use the same set of tones may fail requiring them to retry during a later dedicated registration period, e.g., using another set of tones. As part of the registration process, physical layer signaling issues are resolved such as physical signal timing used to control symbol transmission and/or transmission power control is achieved, e.g., based on control signals received from the new base station. In addition, one or more device identifiers used to identify the device while in the new cell may be assigned to the device seeking to register in the new cell/sector. Once synchronization and ID assignment issues are resolved in regard to the new cell/sector, higher level signaling, e.g., IP packet transmission and reception may begin to occur between the mobile device entering the new sector and/or cell and the base station in that sector/cell.
In the case where the frequency bands of the old and new sector and/or cell are the same, it is often possible to maintain communications with the old base station while simultaneously communicating in the same frequency band with new base station to perform the above discussed registration operations, e.g., timing control, power control and cell/sector ID specific assignment operations. This is possible since the frequency of the filter used in the receiver and/or transmitter need not be changed when communicating with base stations in cases where the old and new carrier frequencies are the same. Thus, in systems where the old and new frequency bands are the same a mobile device can complete physical layer signaling operations which need to be completed before IP packets can be received/transmitted in the new cell while still being able to receive IP packets from the old base station. Once the physical layer, e.g., timing synchronization, etc., with the new base station, and other registration operations are completed in the new sector/cell a signal may be sent to trigger re-routing of IP packets to the mobile device by way of the new sector/cell and to stop the routing of packets intended for the mobile to the old sector/cell. In this way, in various known systems, the connection with the old cell is broken after a connection, sufficient to communicate IP packets, with a new cell or sector, is established.
While using a single carrier which is the same in each sector and cell of a system simplifies handoff operations, it has drawbacks due to the relatively high degree of interference at sector and cell boundaries. At such boundaries, given signal fading, mobile nodes may experience signal conditions considerably worse than 0 dB for extended periods of time.
When different sets of frequencies are used in adjoining sectors/cells, e.g., a frequency reuse pattern greater than 1 is employed, signal conditions at sector and cell boundaries are usually considerably better than in cases where there is full reuse of all frequencies. Thus, signal interference at cell/sector boundaries provides a reason to avoid a frequency reuse scheme of 1 despite the handoff benefits it provides.
Delays associated with adjusting a transmitter and/or receiver's filter to operate at a new frequency band makes switching receiver and transmitter circuitry between an old and a new carrier frequency at a rate that is quick enough to support the above discussed make before break handoff procedure difficult to implement. Accordingly, in handoffs between sectors and/or cells using different frequency bands, a break before make handoff operation is often used where radio signaling with the old base station is terminated before it is established with a new base station. Unfortunately, this normally leaves the mobile device unable to receive IP packets not only during the duration that it is switching its filter circuitry to the new carrier frequency but for the additional time period it needs to register with the new cell/sector and to perform the required timing and/or power synchronization operation(s) and any IP packet redirection operations that may be needed.
The need to wait, in some systems, for a periodically occurring time period during which registrations are permitted to occur within a sector or cell, combined with the uncertainty that resources will be available in the cell or sector for the mobile device to register during a particular registration period, can lead to both unpredictable and sometimes excessive delays before a mobile device can receive IP packets in a new cell or sector after terminating a connection with an old base station.
In view of the above discussion, it should be apparent that there is a need for methods and apparatus for reducing the amount of time required to complete a handoff in a system which uses different frequency bands. It is desirable that at least one or more methods be provided which avoid the need for a mobile device to terminate a connection with a current base station and/or cell before it can commence communication with a new base station or cell in regard to handoff related matters, e.g., registration signaling, assignment of airlink related resources such as local identifier assignments, etc. It is also desirable, that in at least some embodiments, that a mobile device be able to expect with a reasonably high degree of certainty the communications resources needed to complete a registration process will be available at or near the time it terminates communication with a previous base station.